Question

The acidic strength order of alcohol is (A) \(1^{\circ}>2^{\circ}>3^{\circ}\) (B) \(3^{\circ}>2^{\circ}>1^{\circ}\) (C) \(2^{\circ}>1^{\circ}>3^{\circ}\) (D) \(1^{\circ}>3^{\circ}>2^{\circ}\)

Short answer

The correct order of acidic strength is primary (1°) > secondary (2°) > tertiary (3°) alcohols due to the stability of the resulting alkoxide ions after deprotonation, influenced by the increasing electron-releasing effect of alkyl groups. This corresponds to option (A).

Step-by-step solution

Understanding the Concept

Before diving into the possible answers, let's remember the principles that govern acidity. The strength of an acid is determined by the stability of the resulting anion after the acid has donated its proton (Hydrogen ion). In the case of alcohols, the anion is an alkoxide ion. The more stable the alkoxide ion, the stronger the alcohol as an acid.

Consider Inductive and Hyperconjugation Effects

The alkyl groups attached to the oxygen atom in alcohols are electron-releasing both due to the inductive effect (slight shift of electron density through sigma bonds caused by electronegativity differences) and hyperconjugation (delocalization of electrons in π orbital with the adjacent filled p orbital). These two effects increase the electron density on the oxygen leading to a repulsion of electrons on a negatively charged oxygen atom and thus destabilizing the alkoxide ion.

Analyzing Degree of Substitution

In this context, the 1° indicates a primary alcohol which has the hydroxy group (OH) attached to a carbon atom that is itself attached to only one other carbon atom. 2° and 3° indicate secondary and tertiary alcohols, respectively, where the carbon atom with the hydroxy group is attached to two or three other carbon atoms. Now, with the greater degree of substitution, we have an increased number of alkyl groups, which means an intensified electron-releasing effect due to more inductive effect and hyperconjugation. This leads to less stable alkoxide ions.

Selecting Correct Order of Acidic Strength

After recognizing these principles, it's possible to see that the alcoholic strength decreases with increasing the number of alkyl groups attached to the carbon atom with the hydroxy group. So, the correct order of acidic strength of alcohol is primary (1°) > secondary (2°) > tertiary (3°), which corresponds to option (A) of the given exercise. This is founded in the principle that the more alkyl groups present, the less stable the resulting anion will be after proton donation due to an enhanced electron-releasing effect. Consequently, it's less likely to donate a proton and hence be a weaker acid.

Key concepts

Alkoxide Ion Stability

Understanding the stability of alkoxide ions is crucial for grasping why some alcohols are more acidic than others. When an alcohol donates a proton, it forms an alkoxide ion - a negatively charged oxygen atom bonded to an alkyl group. The key to an alkoxide ion's stability is the dispersal of its negative charge. If the negative charge is localized on the oxygen, the ion is less stable and the alcohol is a stronger acid. This is because a strong acid readily releases its proton, leaving behind a stable conjugate base, in this case, the alkoxide ion.

For instance, methoxide ion (CH₃O⁻), which is the conjugate base of methanol, has no alkyl groups which could donate electron density to the oxygen atom. It is relatively more stable as a negative charge is not destabilized by additional electron-releasing alkyl groups. Conversely, as we move to larger alkoxide ions with more alkyl groups, the stability decreases because the electron-donating effects of the alkyl groups increase electron density on the oxygen, causing instability. Thus, the more stable the alkoxide ion, the stronger the corresponding acid.

Inductive and Hyperconjugation Effects

The inductive and hyperconjugation effects are the two primary electron-donating mechanisms in organic molecules that significantly influence acidity. The inductive effect refers to the shifting of electrons in a molecule through sigma bonds due to differences in electronegativity. For instance, alkyl groups, which are less electronegative than oxygen, push electrons towards the oxygen atom, destabilizing the alkoxide ion.

Hyperconjugation is a more subtle effect. It involves the delocalization of electrons in a sigma bond (typically C-H or C-C) to an adjacent p orbital or π system. This delocalization also increases the electron density around the oxygen atom in an alkoxide ion, thereby destabilizing it.

Together, these effects influence the stability of alkoxide ions and hence the acidity of alcohols. Alcohols attached to primary carbons (1°), which have fewer alkyl groups, experience weaker inductive and hyperconjugation effects and thus form more stable alkoxide ions when they donate a proton. This makes them more acidic compared to their secondary (2°) and tertiary (3°) counterparts.

Substitution Effects on Acidity

The number of alkyl groups attached to the carbon bearing the hydroxy group in an alcohol affects its acidity, and this is termed the substitution effect. Primary alcohols have one alkyl group, secondary alcohols have two, and tertiary alcohols have three, which influence the strength of the alcohol as an acid.

As the degree of substitution increases from primary to tertiary, the number of electron-donating alkyl groups attached to the hydroxy-bearing carbon increases. This means that there is a larger inductive effect and greater hyperconjugation as explained earlier, leading to a higher electron density on the oxygen atom in the alkoxide ion. This destabilization makes the alkoxide ion less likely to accept the negative charge that comes with losing a proton, which corresponds to a decrease in acid strength.

Therefore, the acidity of alcohols decreases as the substitution from primary to tertiary increases: primary (1°) > secondary (2°) > tertiary (3°). This trend highlights the significance of both inductive and hyperconjugation effects in determining the stability of alkoxide ions and the overall acidic strength of alcohols.